The basic process for preparing methyl chlorosilanes is the direct reaction of milled silicon with methyl chloride in the presence of copper as catalyst. The reaction is known to a person skilled in the art as the "Rochow synthesis" and is described in U.S. Pat. No. 2,380,995.
According to this process, a mixture of methyl chlorosilanes in which dichlorodimethylsilane (Di) is the main constituent is formed. In addition, methyl trichlorosilane (Tri) and other products such as, for instance, trimethyl chlorosilane (mono), tetramethyl silane (TMS), methyl hydrogen dichlorosilane (MeH) and higher boiling methylchlorodisilanes (PS) are formed.
Since the discovery of this synthesis, efforts have been made to improve the performance of the synthesis and to increase the proportion of dichlorodimethylsilane.
This is achieved in particular by using purer raw materials and by the use of promoters. Suitable promoters, according to U.S. Pat. No. 4,602,101, are zinc, tin and phosphorus, as the elements or in the form of their compounds. U.S. Pat. No. 5,059,706 also discloses, in addition to zinc and optionally tin, the use of volatile phosphorus compounds as promoters. At high phosphorus concentrations, although acceptable selectivities with respect to dichlorodimethylsilane are obtained, the yields are unsatisfactory.
The object of the present invention was, therefore, the provision of a process for preparing alkyl chlorosilanes which has a high selectivity and a high yield.